Worst Case Analysis (WCA) is typically performed to evaluate whether a circuit continues to function properly when subjected to part variations due to environmental extremes including (but not limited to) radiation, temperature, and aging given the possible parameter tolerances associated with the parts in the circuit design. WCA typically involves constructing mathematical models to describe the behavior of the circuit functions and to verify that the circuit performs within specification. There are several different approaches to WCA, but one convenient approach to performing WCA is Extreme Value Analysis (EVA). EVA is usually the least complicated and most conservative approach to WCA. As such, it is a good initial approach. If the circuit passes, then there is very high confidence in the results and the minimum amount of work was required to verify it. If the circuit does not pass, then the design or requirements can be modified or a less conservative, but more difficult, WCA approach can be used.
When performing WCA using the EVA methodology, the parameters of the parts within each (sub)circuit which affect performance are combined such that each environment that drives them to their extrema is simultaneously acting on the part. For example, in a space mission, if an operational amplifier's offset voltage is maximized when temperature is low, radiation is high, and at the end of the mission, then that combination of conditions (minimum temperature, maximum radiation, and end of mission) is assumed in calculating the maximum offset voltage. Usually, parameter changes are multiplied assuming that they are independent influences on the parametric behavior. Thus, to determine the maximum positive fractional variation for a parameter:(1+dP)=(1+dX)(1+dS)(1+dT)(1+dE)(1+dR)
where:                dP is the total parametric variation;        dX is the part initial tolerance;        dS is the variation due to aging and drift;        dT is the variation due to temperature (worst-case direction);        dE is the variation due to applied voltage and frequency; and        dR is the variation due to radiation degradation.        
If another set of conditions on the aforementioned operational amplifier leads to maximum bias current for that operational amplifier, then those conditions are assumed in calculating the maximum bias current. In general, a Worst Case Database (WCDB) can be generated one time for the parameters of the parts to be used on all the subsequent EVAs.
In many applications, two sets of WCAs are performed, or at least considered; one for conditions with the circuit powered (i.e., biased) and one for conditions when the circuit is unpowered (i.e., unbiased). Since radiation degradation for some parts is worse unbiased, that case must be considered. Redundancy is of no value, if a circuit will not turn on when it is eventually needed. Alternatively, the variations in the WCDB may include the worse of biased and unbiased performance, and the analyst would not have to consider biased/unbiased differences.
Considering the extreme radiation environment that can be experienced in many applications and the additional part parameter variations that can result from high radiation, it is likely that many circuits or portions of circuits designed for such applications will not pass EVA. If the circuit does not pass, then the design can be modified or a less conservative, but more difficult, WCA approach can be used; for example, temperature tracking is often tried as a modification to EVA to attempt to “sharpen the pencil” and get a WCA to pass. The intent in doing a different approach is to more accurately assess the margins in the design even though more work is required. WCA should always be a rigorous (i.e., conservative) assessment of the circuit's expected performance; the intent should not be to get around the process in order to just get the analysis to pass.